Power supply with a reduced harmonic load on the mains system, and a corresponding appliance

ABSTRACT

The power supply according to the invention has a first switched-mode power supply and a second switched-mode power supply, which is connected in parallel with the first, and with both switched-mode power supplies having power factor correction. In this case, the first switched-mode power supply has an active power factor correction circuit, for example with a current pump which contains a coil which is coupled to a primary winding of a transformer, and the second switched-mode power supply has a power factor coil (NS) in its power supply for power factor correction. Both switched-mode power supplies are in this case arranged in one appliance, in particular in parallel and downstream from a common mains switch. The active power factor correction in this case flattens the pulsed current flow, and the power factor coil shifts the phase of the pulsed current flow. Since the current flows in the switched-mode power supplies, which are connected in parallel, are added to one another, this addition results in additional broadening of the pulsed current flow, which leads to a further improvement in the power factor for the power supply, and/or the inductance of the power factor coil can in consequence be chosen to be correspondingly lower.

This application claims the benefit under 35 U.S.C. § 365 ofInternational Application PCT/EP02/02239 filed Mar. 1, 2002, whichclaims the benefit of German Application No. 101 62491.3 filed Dec. 19,2001 and German Application No. 10113297.2 filed Mar. 16, 2001.

BACKGROUND OF THE INVENTION

The present invention is based on a power supply having twoswitched-mode power supplies. The first switched-mode power supplycomprises in particular a current pump with a coil for an active powerfactor correction and the second switched-mode power supply comprises apower factor coil in its current supply.

It is known for entertainment electronic appliances to use twoswitched-mode power supplies to supply power to the very wide range ofcircuit components, which switched-mode power supplies are connected inparallel in order to provide the necessary number of supply voltages,and/or in order to make it possible to switch off specific circuitgroups as a function of a chosen operating mode. In television sets, forexample, which have an Internet module or a digital decoder, it isworthwhile using a separate switched-mode power supply for this circuitunit, which separate switched-mode power supply is switched on and offas required, with this unit.

Switched-mode power supplies result in a highly pulsed load on the linenetwork, which leads to harmonic currents in the line network. This loadoccurs in the region of the voltage maximum of the sinusoidal mainsvoltage, in which region an energy storage capacitor in theswitched-mode power supply is recharged. Appliances having a relativelyhigh power consumption, such as relatively large television sets, musttherefore now satisfy specific regulations with regard to the harmoniccurrents. The harmonic load on the line network from an appliance can inthis case be specified by a so-called power factor.

It is known for a so-called “preconverter” to be used for active powerfactor correction in order to reduce the harmonic load in the linenetwork, which preconverter is connected upstream of the switched-modepower supply and has a coil to which a current is applied cyclically bya switch. This results in a quasi-continuous current flow from the linenetwork.

Furthermore, switched-mode power supplies are known which use powerfactor correction with a so-called current pump, for example fromDE-A-196 10 762, EP-A-0 700 145 and U.S. Pat. No. 5,986,898. These havea first current path on the primary side, through which an energystorage capacitor in the switched-mode power supply is charged via adiode and a mains rectifier in the switched-mode power supply, and havea second current path with an inductance which is arranged between themains rectifier and the primary winding of the transformer. The energystorage capacitor is in this case charged predominantly via the secondcurrent path. The inductance of the second current path thus acts like acurrent pump which, controlled by the switching transistor, draws acontinuous current or at least a broadened current flow from the linenetwork, in order to improve the power factor. In this sense, theseappliances are thus also switched-mode power supplies with active powerfactor correction since, in the case of these switched-mode powersupplies as well, an at least broadened current flow is produced fromthe switched-mode power supply, with an improved power factor, via acoil and by means of a transistor, in this case the switching transistorof the switched-mode power supply.

A further switched-mode power supply with active power factor correctionis known from EP-A-0 588 168, EP-A-0 588 172 and EP-A-0 588 173. Thislikewise has two current paths, with the first current path connectingthe bridge rectifier via a diode directly to the primary winding of atransformer. The primary winding thus operates directly with therectified mains voltage. An energy storage capacitor is arranged in thesecond current path and is discharged via a second switching transistoras a function of the first switching transistor, in order to improve theripple on the output voltages from the switched-mode power supply.

It is also known that it is possible to improve the power factor bymeans of a coil which is arranged at the input of the switched-modepower supply, in particular upstream of the energy storage capacitor.This coil is also known as a mains frequency coil, 50 Hz coil or powerfactor coil. In order to avoid confusion with other coils, the termpower factor coil is used throughout the description here.

However, this power factor coil requires a relatively large amount ofinductance to achieve sufficiently good power factor correction. Afurther disadvantage is that, when the mains switch is operated in orderto switch the appliance off, the current flow in the power factor coilis interrupted suddenly. The energy stored in the power factor coilmust, however, be dissipated. Since the open mains switch represents thehighest impedance in the circuit, a very high voltage is thereforedeveloped across the switching contacts of the mains switch, leading toarcing. Depending on the configuration of the switch, the rate at whichthe switch opens may also be comparatively slow, so that the arc isproduced until the end of that mains half-cycle. In this case, no highvoltage is formed across the contacts, but, in both cases this meansthat the mains switch ages more quickly, and the switch represents asafety risk since, in the worst case, it becomes a potential firesource.

If an appliance has two switched-mode power supplies, but only one mainscable which connects the two switched-mode power supplies to the linenetwork, then the appliance is regarded as a unit which must comply withthe regulations for power factor correction.

SUMMARY OF THE INVENTION

The power supply according to the invention has a first switched-modepower supply and a second switched-mode power supply, which is connectedin parallel with the first, and which each have their own power factorcorrection. The first switched-mode power supply has an active powerfactor correction circuit, and the second switched-mode power supply hasa power factor coil in the power supply to the second switched-modepower supply for power factor correction. The active power factorcorrection in this case flattens the pulsed current flow, and the powerfactor coil shifts the phase of the pulsed current flow. Bothswitched-mode power supplies are in this case arranged, in particular inparallel, downstream from the mains switch of an appliance.

This results in a power supply having the following surprisingadvantages: the problem of excessively loading the mains switch now nolonger exists, since the energy which is stored in the power factor coilis now dissipated via the energy storage capacitor of the firstswitched-mode power supply when the mains switch is switched off, sincethe two switched-mode power supplies are connected in parallel, that isto say the mains switch is not loaded by the power factor coil onswitching off. This leads to the appliance having high reliability,while also using a power factor coil.

Furthermore, the two different principles for power factor correctionlead to construction addition of the two current flows: the active powerfactor correction with a current pump leads to flattening of the pulsedcurrent flow, that is to say the current flow angle is broadened bothbefore and after the voltage maximum of the mains voltage. The pulsedcurrent flow of the second switched-mode power supply is, however,shifted in phase by the power factor coil with a lagging phase, so thatit leads mainly to broadening of the current flow angle after thevoltage maximum of the mains voltage. Since the two current flows of theswitched-mode power supplies, which are connected in parallel, are addedto one another, this addition results in an additional improvement inthe power factor for the power supply. The inductance of the powerfactor coil can in consequence be chosen to be less by a factor of twothan would actually be necessary for the power of the secondswitched-mode power supply.

The combination of the two different power factor correction circuits isthus an optimum configuration for an appliance which contains a powersupply with two switched-mode power supplies, and involves only a smallamount of complexity for the second switched-mode power supply. Theinvention is applicable in particular to television sets with digitalcircuits, and to other entertainment electronic appliances which have arelatively high power consumption.

The invention will be explained in more detail in the following textusing, by way of example, an exemplary embodiment which is illustratedschematically in the figures, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a circuit diagram of a first switched-mode power supplywith an active power factor correction circuit;

FIG. 2 shows a second switched-mode power supply with a power factorcoil for power factor correction, and

FIG. 3 shows the resultant currents and voltages from the twoswitched-mode power supplies shown in FIGS. 1 and 2.

DETAILED DESCRIPTION OF THE DRAWINGS

FIG. 1 shows, in simplified form, a switched-mode power supply I with amains rectifier BR1, an energy storage capacitor C20, a switchingtransistor T1 and a transformer Tr1, which has a primary winding W1 andsecondary windings W2-W4. The mains rectifier BR1 has an inputconnection BP2, via which the rectifier is connected to a mains switch(not shown) of a corresponding appliance and to the line network, and isused for rectification of the mains voltage. The output from the mainsrectifier BR1 is connected via two current paths I1 and I2 to thetransformer Tr1: it is connected via the first current path I1 to aconnection 1 of the primary winding W1 and to the energy storagecapacitor C20, and is connected via the current path I2 to a tap A onthe primary winding W1. The current path I1 has a diode D16, whichprovides decoupling between the second current path I2 and the energystorage capacitor C20.

The current path I2 has a coil L19 which results in active power factorcorrection, and a capacitor C19 for current limiting in the current pathI2. A diode can also be used in the current path I2 instead of thecapacitor C19. The tap A may also be identical to the connection 1 or 2of the primary winding W1. A resistor R10 which is connected downstreamfrom the mains rectifier BR1 likewise provides current limiting, in thiscase for both current paths I1 and I2.

The transformer TR1 produces, in a known manner, mains isolation betweenthe primary side and the secondary side of the switched-mode powersupply. The switching transistor T1, which is connected in series withthe primary winding W1, is driven by a driver stage (not shown) withswitching signals DS1, resulting in power being transmitted to thesecondary windings W3 and W4 which are arranged on the secondary side,as is known. The switched-mode power supply in this case operates on theprinciple of a flyback converter, as is frequently used forswitched-mode power supplies in entertainment electronic appliances.

In this case, the switched-mode power supply operates as follows: oncethe appliance has been switched on, the energy storage capacitor C20 isfirst of all charged via the resistor R10 and the current path I1. Theresistor R10, which has a small resistance of, for example, 2.7 ohms, inthis case limits the current flow. The energy storage capacitor C20 mustbe charged for the starting phase of the switched-mode power supply.During normal operation, the switching transistor T1 essentially draws acurrent via the current path I2, through which the energy storagecapacitor C20 is then recharged via a diode D18 and via the primarywinding W1.

Ideally, the inductance of the coil L19 in this case results in acontinuous current flow from the line network, which leads to acorresponding improvement in the power factor. A coil L15 which isconnected upstream of the coil L19 in this case prevents theradio-frequency switching pulses of the switched-mode power supply frompropagating via the bridge rectifier BR1 into the power network. Thecapacitors C10, C11 and C12, which have only a small capacitance, areused to filter the mains voltage or, likewise, for suppression.

The switched-mode power supply has a regulator for stabilizing theoutput voltages and, using, for example, pulse-width modulation of thedriver signals DS1, as a function of the load, for regulation. Since thecurrent flow I2 is controlled by the switching transistor, the energystorage capacitor C20 is thus also recharged as a function of the loadon the switched-mode power supply. Further details relating to theoperation of this switched-mode power supply and, in particular,relating to active power factor correction are described, for example,in U.S. Pat. No. 5,986,898, and will thus not be explained any furtherhere.

FIG. 2 shows, schematically, a second switched-mode power supply II,which has a power factor coil NS for passive power factor correction.The power factor coil NS is in this case arranged between a mains filterNF and a mains rectifier BR2 in the switched-mode power supply II. Anenergy storage capacitor C30 and the primary winding W5 of thetransformer Tr2 are connected to the output of the bridge rectifier BR2.Like the transformer Tr1, the transformer Tr2 has secondary windingsW6-W9, and its primary winding is likewise connected in series with aswitching transistor T2, which is controlled by a driver circuit DC.

A supply voltage VCC for the driver circuit DC is in this case providedvia the winding W6 arranged on the primary side, and output voltages U1,U2, U3 for supplying corresponding circuits are provided via thesecondary windings W7-W9 arranged on the secondary side. The drivercircuit DC in this case regulates the output voltages U1-U3 of theswitched-mode power supply with the aid of a regulating signal RS, whichcan be derived on the primary side or secondary side from one of thesecondary windings of the transformer TR2.

The switched-mode power supply II also has a starting circuit AS forstarting the switched-mode power supply, and a so-called snubber networkSN2, which is used to damp switching spikes when the switchingtransistor T2 switches off. The power factor coil NS, which is locatedin the current path of the 50 Hz mains frequency upstream of the bridgerectifier BR2, has an inductance of, for example, 20 mH, so that thecharging process for the energy storage capacitor C30 has a considerabledelay. Apart from the power factor coil NS, the switched-mode powersupply 2 is thus identical to already known flyback converters withoutpower factor correction, and it has only one current path, via which theenergy storage capacitor C30 is charged. There is no need for anyfurther components for power factor correction here and, in particular,there is no need to use a transformer with a tap. The switched-modepower supply thus operates together with the power factor coil NS in acorresponding way to a flyback converter without power factorcorrection, with the difference that the energy storage capacitor C30 isrecharged with a current pulse which is broadened and delayed by theinductance of the power factor coil.

The input of the switched-mode power supply II has a connection BP1which leads to the connection BP2 of the first switched-mode powersupply I, and a connection UN, which is connected to the line networkvia the mains switch and the mains cable of an appliance, which is notillustrated. The switched-mode power supply II is in consequenceconnected to the mains voltage in parallel with the switched-mode powersupply I. The switched-mode power supply II is in this case the smallerswitched-mode power supply in the appliance, and can be switched on andoff by a control signal from the secondary side, not shown, depending onthe operating mode of the appliance.

The input currents which occur during operation of the two switched-modepower supplies I and II are shown in FIG. 3. The input current of theswitched-mode power supply 1 is in this case represented by the curve 3,and the input current of the switched-mode power supply 2 is representedby the curve 4, while the mains voltage that is applied to the applianceis represented by the curve 1. The resultant current flow for the powersupply is represented by the curve A. As can clearly be seen from thecurve 3, the switched-mode power supply I draws a relatively constantcurrent in a region around the voltage maximum of the mains voltage,curve 1. The current has no phase shift with respect to the mainsfrequency or mains voltage.

The current flow from the second switched-mode power supply II curve 4,on the other hand, has a gradual rise with a maximum which occurs at atime after the maximum mains voltage. Owing to the different currentcharacteristics of the two switched-mode power supplies I, II, theaddition of the two currents thus leads to additional broadening of thecurrent flow, in comparison to a power supply which would use only onecircuit principle for power factor correction. The power factor coil NSin the switched-mode power supply II already has a correspondinglyreduced inductance in this case than would be necessary for theswitched-mode power supply II just to satisfy the power factorcorrection Standard.

This is illustrated by an example: if the switched-mode power supply IIhas a mains power consumption of 100 W, then, if it is connected on itsown as the sole switched-mode power supply to the mains system, itrequires a power factor coil with an inductance of 40 mH to satisfy thepower factor correction Standard. However, if this switched-mode powersupply is connected in parallel with the switched-mode power supplywhich likewise has a mains power consumption of 100 W and containsactive power factor correction with a current pump, then the inductanceof the power factor coil need be only 20 mH for the two switched-modepower supplies, as a power supply, to comply with the limit values inthe power factor correction Standard. The power factor coil in thesecond switched-mode power supply is thus smaller and cheaper.

A cost-effective power supply with two switched-mode power supplies canthus be produced by using one switched-mode power supply with activepower factor correction, in particular using a current pump, and with asecond switched-mode power supply which has a power factor coil shiftingthe phase of the input current, which power supply does not excessivelyload the mains switch in an appliance and in which, furthermore, a powerfactor coil with a reduced inductance can be used in the secondswitched-mode power supply. The principle can also be applied to threeswitched-mode power supplies connected in parallel, in which case apower factor coil can then likewise be used for the third switched-modepower supply.

Further refinements of the invention will be familiar to a personskilled in the art. The invention is not restricted to flybackconverters and can likewise be used for other switched-mode power supplyconcepts, for example in switched-mode power supplies which have two ormore switching transistors. In particular, the switched-mode powersupply II is not restricted to flyback converters since, when using apower factor coil for power factor correction, nothing need be changedin the design of a switched-mode power supply.

1. Power supply having a first switched-mode power supply and a secondswitched-mode power supply, both being connected in parallel supply andboth having a common mains voltage connection providing a pulsed currentflow, wherein said first switched-mode power supply comprises a currentpump with a coil for an active power factor correction, the coil beingcoupled to a primary winding of a transformer of said firstswitched-mode power supply, and said second switched-mode power supplycomprises a power factor coil in its current supply in order to shift aphase of the pulsed current flow with regard to the maximum of the mainsvoltage.
 2. Power supply according to claim 1 wherein the firstswitched-mode power supply comprises a first current path with a diodefor charging an energy storage capacitor and a second current path witha coil for producing a connection between a rectifier and a terminal ofthe primary winding of a transformer.
 3. Power supply according to claim1 wherein the power factor coil is arranged upstream of the mainsrectifier of the second switched-mode power supply.
 4. Power supplyaccording to claim 3, wherein the inductance of the power factor coil isin a range from 1-30 mH.
 5. Power supply according to claim 1 whereinthe second switched-mode power supply has a lower output power, incomparison with the first switched-mode power supply.
 6. Power supplyaccording to claim 1 wherein both switched-mode power supplies are inthe form of flyback converters with mains isolation via a transformer,and in that the second switched-mode power supply can be switched on andoff independently of the first switched-mode power supply.